High-performance liquid chromatography (HPLC) is an efficient, innovative, fast, and widely used analytical technique. HPLC has already become one of the most commonly used analytical tools in chemistry, food hygiene, drug testing, environmental monitoring, and many other areas. Packing material surface property will influence seriously the retention process of the analytes in LC columns. The chemistry and structure of the packing material control the specific polar and non-specific solvophobic interactions between the analyte and the stationary phase, which decide on the separation mode in HPLC.
Packing materials for HPLC are generally classified into three types: organic polymeric materials, e.g., polydivinylbenzene, inorganic materials typified by silica, and hybrid materials. Many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials shrink and swell when the composition of the mobile phase is changed. In addition, most organic chromatographic materials do not have the mechanical strength of typical chromatographic silicas.
Among the inorganic materials, which include silica, hydroxyapatite, graphite, and metal oxides, etc., silica is almost an ideal support in view of its favorable characteristics, for example good mechanical strength, high chemical and thermal stability, controllable pore structure and surface area, surface rich in silanol groups, etc. Silica-based stationary phase can offer high chromatographic efficiency and reproducible separations over a wide range of operating conditions. In particular, their extremely rich silanol surface chemistry endows silica packings flexible chemical surface tailorability, i.e., various kinds of organic groups such as octadeyl, octyl, phenyl, amino, cyano, etc. could be tightly grafted onto the silica surface, and have accordingly been applied in different separation modes. However, during a typical derivatization process such as reaction with octadecyldimethylchlorosilane, at least 50% of the surface silanol groups remain unreacted. The surface silanol groups of silica can be attacked under alkaline conditions, leading to the dissolution of silica and column degradation. In most cases, silica-based packings, including bare silica, and bonded phase, have not been recommended to be used above pH 8. Additionally, these residual silanol groups interact with basic and acidic analytes via ion exchange, hydrogen bonding and dipole/dipole mechanisms. The residual silanol groups create problems including increased retention, excessive peak tailing and irreversible adsorption of some analytes.
To overcome the problems of residual silanol group activity and high pH stability of silica-based stationary phases, many methods have been tried including post-synthesis grafting approaches and direct synthesis approaches. Post-synthesis modification of silica is mainly focused silane coupling agents to react with more Si—OH groups on the surface and shield residual surface silanol. For example, end-capped bonded-phase, bidentate silane, and horizontal polymerization technology have been successfully taken to extend the high pH stability of silica-based phases. Porous hybrid silica materials with organic units distributed homogeneously in the framework as direct components exhibit improved stability in high-pH mobile phases. However, the introduction of organic units in the surface and framework of hybrid silica restricts the use of such hybrid silica materials as hydrophilic stationary phase. Thus, the quest to develop stationary phase material with high-pH stability and flexible chemical surface tailorability is of extreme significance.
Silicon oxynitride material has attracted a great deal of attention as nitrogen-containing solid basic material. By treating porous silica precursor under ammonia condition at high temperature, nitrogen or NHx species such as NH2 and bridging NH groups can be incorporated into the framework of silica through displacement of silanol groups and bridging oxygen without compromising the characteristics of silica (morphology, high surface area, and narrow pore size distribution). With polar surface NHx groups and a porous structure, this porous silicon oxynitride material is stable to alkaline mobile phases. The reactivity of surface Si—NH2 and Si—NH—Si groups of porous silicon oxynitride material allows tailoring of the surface through modification using different functionalized reagents, and opens up a broad applicability in different separation modes.